Color-plus-clear composite coatings are widely utilized in the coatings art. Color-plus-clear composite coatings are particularly desirable where exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of color-plus-clear composite coatings for automotive body panels. Such coatings, however, require an extremely high degree of clarity in the clearcoat to achieve the desired visual effect. There is a continuing need in the art for the development of clearcoats for composite coatings that will exhibit the desired clarity for long periods of time under exposure to a variety of conditions.
Clearcoats utilizing a carboxy-epoxy cure mechanism or an anhydride-epoxy cure mechanism have been proposed for composite coatings. These clearcoats are described, for example, in U.S. Pat. Nos., 4,650,718; 4,681,811; 4,703,101; and 4,732,790. This type of curing depends on a reaction between a compound having epoxy-functional group(s) and a curing agent having carboxy-functional group(s) or anhydride-functional group(s). By way of an oxirane ring-opening reaction between the carboxy or anhydride groups and the epoxy groups, a matrix of chemical bonds is formed, resulting in a hard cured coating.
Basecoat coating compositions used in color-plus-clear composite coatings utilize a polymeric component and a curing agent. The polymeric component typically has active hydrogen-containing groups (e.g., hydroxyl) as pendant or terminal groups on a polymeric backbone (e.g., polyurethane or acrylic). The curing agent is typically an aminoplast such as melamine formaldehyde resin, which may be partially or fully alkylated. Coating compositions that rely on aminoplast curing will also usually contain a strong acid catalyst. Commonly-used catalysts include aromatic sulfonic acids (e.g., p-toluenesulfonic acid, dinonylnaphthalenedisulfonic acid, dodecylbenzenesulfonic acid), phenyl acid phosphate, monobutyl maleate, butyl phosphate, and hydroxy phosphate ester.
Basecoat coating compositions such as those described above containing acid cure catalysts often utilize an amine group-containing compound to improve the stability and/or shelf life of the coating composition. Amine group-containing compounds used in the art are typically lower alkyl tertiary amines or certain heterocyclic amines such as oxazolidine. It is believed that the amine compound complexes with the acid catalyst, thus preventing premature onset of the curing reaction. Upon heating during cure, the amine complex dissociates, liberating the acid functionality to act as a cure catalyst.
However, when basecoat compositions stabilized with amine group-containing compounds as described above are used in combination with clearcoats utilizing a carboxy-epoxy or anhydride-epoxy cure mechanism, severe defects can result during curing. Such defects can be described as a pop, blister, crater, or blowout. The defects are often characterized by a discontinuity in the coverage of the clearcoat that may be surrounded by material from the basecoat and/or clearcoat deposited onto the otherwise smooth surface of the clearcoat.
There is thus a need in the art for color-plus-clear composite coatings that provide the desired appearance characteristics while avoiding the above-described problems.